The present invention relates to a flexible housing for use in protecting one or more transmission lines (e.g. microwave cables, radio frequency cables, flexible wave guides, fibre optic cables, DC lines, high/low pressure air or hydraulic hoses and other transmission media) in a pressurised environment and in particular, a hydrostatically pressurised environment.
The invention originates from a requirement for a microwave transmission line to operate within a submarine mast environment. Modern submarine mast design dictates the placement of electronic equipment in an electronics pod at the mast head connected by microwave transmission lines to processing equipment in the hull. This arrangement creates the requirement for a microwave transmission line capable of withstanding a hydrostatic pressure of up to 1500 psi (1.03.times.10.sup.7 pascals) and yet able to flex through an angle of 180.degree. with a bend radius of 150 mm when the mast head is raised.
In the past various "wet mast" techniques have involved-the ruggedising of coaxial microwave cables so as to limit to an acceptable level the amount of hydrostatic pressure felt by the cable. Often under such pressure conditions it is the dielectric within the coaxial cable that absorbs the bulk of the compressive forces, altering the microwave characteristics of the cable. The use of a more rigid dielectric material to overcome the problem of compression under hydrostatic pressure has an adverse affect on cable performance, furthermore, this technique limits the types of cable which can be used, their number and their application and, consequently, has a limiting and detrimental affect on the design and performance of the system as a whole.
The above technique also requires the use of some form of cable management system to ensure that the cables hang and remain in a defined position (referred to as the "dip loop") under the various dynamic conditions of the submarine. Each transmission line linking the electronics pod at the mast head with the processing equipment in the hull will contain a flexure through 180.degree., the point of flexure being determined by the relative heights of the terminations of the transmission line and the point of flexure will thus change as the mast is raised. The cable management system not only has to allow for the smooth change in position of the point of flexure but also has to ensure that the transmission lines are unaffected by the angle of dive of the submarine, which may be of the order of .+-.10.degree., or the angle of tilt of the submarine caused by a surface swell and possibly of the order of +30.degree..
Since the degree of bending of the cable may of itself bring about changes in the electrical characteristics of the cable, it is desirable that the bend radius be maintained constant as far as possible.
Our published patent applications GB2219439 and 2219440 disclose flexible housings formed of individual segments, where the segments are provided with abutments to limit the degree of bending of the housing. This is desirable to avoid bending too sharply the microwave transmission lines travelling through the housing. In particular, GB2219440 discloses a flexible housing formed of alternating biconcave and cylindrical members, where abutments are provided on the leading edges of the bi-concave segments to allow for limited bending in one direction only away from the straight orientation. However, as the housing flexes, there is a danger that the jacket surrounding the segments may become pinched between the abutting surfaces of adjacent segments which may lead to damage to the jacket and possible jamming of the housing.
It is an object of the present invention to mitigate this problem, and to provide a housing capable of withstanding larger flexural forces.